Quality requirements of reticle-forming silica glass substrates include the size and density of defects on substrates, the flatness and surface roughness of substrates, the photochemical stability of material, and the chemical stability of surface. Of these, the quality requirements concerning the on-substrate defects become more stringent on account of the recent trend toward further miniaturization of integrated circuits.
The defective quality of reticle-forming silica glass substrates has been gradually improved. Currently, substrates in which concave defects with a size of less than about 0.3 μm are present are used. This is because the visual inspection under a collective lamp at a sufficient illuminance to avoid any hygienic problem and the defective inspection by an automatic defect detecting system have the technical limit that concave defects such as flaws are less sensitive than convex defects such as particles, and the probability to detect concave defects with a size of less than about 0.3 μm is very low. This inspection limit caused a delay in devising substrate quality improvements.
Even in the substrates in which concave defects with a size of less than about 0.3 μm are apparently absent, chemical reagents used in cleaning and otherwise processing steps can cause enlargement of ultrafine concave defects and release of residual strains, giving rise to the problem that latent damages manifest themselves as defects.
In the case of phase shift masks which are expected to become predominant in the next generation technology, the step of positively etching the silica glass surface is involved. It is problematic if latent damages on the substrate surface manifest themselves during the step.
No substantial attention has been paid to the latent damages in quartz substrates which manifest themselves under the action of reactive reagents. Heretofore, polished substrates are regarded acceptable if no defects are found on the substrate surface. This is one of factors that make indefinite the cause for a substantial rejection rate of reticles.